Progressive cavity pumps have long been used downhole for pumping wellbore fluids. When a lone progressive cavity pump is operated in a well where free gas or foamy oil is present, the pump does not operate as efficiently and its run life decreases. To solve this problem, a pump assembly is used having a charge pump in addition to the main production pump. Referring to FIGS. 1A and 1B, the main pump 106, located at the top of the pump assembly 100, is a high pressure/low volume pump capable of pushing the wellbore fluid the full length of the wellbore. The main (progressive cavity) pump 106 includes a rotor 104, driven by a rotating shaft 102, that turns inside a stator 108 at a fixed rate. The shaft 102 is typically driven by an electric motor (not shown). The charge pump 116, being a low pressure/high volume pump, is located below the main pump 106 and feeds it with wellbore fluid through an interconnect 110, such as a pup joint. The charge pump of FIGS. 1A and 1B is also a progressive cavity pump with a rotor 118 and stator 114, but some pump assemblies may instead use an auger for the charge pump. The effect of adding the charge pump 116 is that, due to the higher pressure in the interconnect 110, the gas is compressed to occupy less volume and may be forced into solution thereby increasing the liquid efficiency of the main pump.
The ratio of the displacements of the pumps in the pump assembly is typically designed according to the gas content of the wellbore fluid, with the charge pump having a higher displacement. When the wellbore fluid has a free gas rate of under 25 percent, a ratio of displacements of approximately 2:1 is commonly employed. With a free gas rate of 25 to 50 percent, the ratio may be approximately 4:1. For example, with free gas in the well at 15 percent, the pump assembly may use a pump with a 100-barrels-per-day displacement as the main pump and a pump with a 200-barrels-per-day displacement as the charge pump.
Problematically, the free gas rate of the wellbore fluid is often non-uniform. When the gas content of the wellbore fluid falls below the range for that the system was designed, the pressure increases dramatically, damaging the charge pump. When the gas content of the wellbore fluid exceeds the anticipated range, the pressure decreases, the effect of the charge pump on the pump assembly is nullified, and the pump assembly becomes inefficient. Non-uniform inflow of water or high viscosity liquids can have the same effect.
A current solution to high-pressure events is to create pressure relief ports 120 in the interconnect 110 in various sizes and configurations. While simple ports can discharge pressure from the interconnect 110, they are inflexible in response to pressure increases in that the amount of fluid and gas discharged from a set number and configuration of ports is proportional to the pressure in the interconnect. These ports 120 also exacerbate the problem of pressure decreases.
FIG. 2A is a graph showing the pressure in the interconnect of the main pump 202 in comparison with the fluid viscosity 216 of the liquid being pumped. Curves representing the pressure in the interconnect 202 for each fluid viscosity 216 are shown for an interconnect alternately having zero (204), two (206), four (208), eight (210), and sixteen (212) ports. As is apparent from FIG. 2A, the greater the number of ports, the more slowly the interconnect pressure 202 increases in comparison to the fluid viscosity 216. In the current ported interconnect method, therefore, using a larger number of ports to avoid a pressure increase detrimental to the charge pump results in a less than optimal range of fluid viscosities that produce an interconnect pressure 202 greater than the minimum of the efficient range 214, and vice versa.
FIG. 2B is a graph showing the pressure in the interconnect of the main pump 234 in comparison with the free gas rate 236 of the liquid being pumped. Curves representing the pressure in the interconnect 234 for each free gas rate 236 are shown for an interconnect alternately having zero (224), two (226), and four (228) ports. As is apparent from FIG. 2B, the greater the number of ports, the more slowly the interconnect pressure 202 increases as the free gas rate 236 decreases. Again, using a larger number of ports to avoid a detrimental pressure increase results in a less than optimal range of free gas rates that produce an interconnect pressure 234 greater than the minimum of the efficient range 214.
The pressure curves of FIGS. 2A and 2B are for example only, as the curves associated with a specific implementation of pump assembly (with varying main and charge pump displacements, sizes of interconnect, sizes and numbers of ports, etc.) will vary.
Changing the port configuration or the displacement from the charge pump when the pressure is approaching the upper or lower limit of the efficient range reduces non-uniformity in interconnect pressure. An ideal design, therefore, would include a mechanism for changing the port configuration or the configuration of the charge pump in response to the pressure at the inlet port of the main pump. Disclosed herein are pump assemblies that include these pressure control mechanisms.